Doped semiconductor nanocrystal layers and preparation thereof

ABSTRACT

The present invention relates to a doped semiconductor nanocrystal layer comprising (a) a group IV oxide layer which is free of ion implantation damage, (b) from 30 to 50 atomic percent of a semiconductor nanocrystal distributed in the group IV oxide layer, and (c) from 0.5 to 15 atomic percent of one or more rare earth element, the one or more rare earth element being (i) dispersed on the surface of the semiconductor nanocrystal and (ii) distributed substantially equally through the thickness of the group IV oxide layer. The present invention also relates to a semiconductor structure comprising the above semiconductor nanocrystal layer and to processes for preparing the semiconductor nanocrystal layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. Provisional Patentapplication Ser. No. 60/441,413, filed Jan. 22, 2003, entitled“PREPARATION OF TYPE IV SEMICONDUCTOR NANOCRYSTALS DOPED WITH RARE-EARTHIONS AND PRODUCT THEREOF”, the contents of which are hereby incorporatedby reference.

FIELD OF THE INVENTION

[0002] The present invention relates to semiconductor nanocrystal layersdoped with rare earth elements, to semiconductor structures comprisingthese semiconductor nanocrystal layers, and to processes for preparingthe semiconductor nanocrystal layers doped with rare earth elements.

BACKGROUND OF THE INVENTION

[0003] Silicon has been a dominant semiconductor material in theelectronics industry, but it does have a disadvantage in that it haspoor optical activity due to an indirect band gap. This poor opticalactivity has all but excluded silicon from the field of optoelectronics.In the past two decades there have been highly motivated attempts todevelop a silicon-based light source that would allow one to havecombined an integrated digital information processing and an opticalcommunications capability into a single silicon-based integratedstructure. For a silicon-based light source (silicon Light EmittingDiode (LED)) to be of any practical use, it should (1) emit at atechnologically important wavelength, (2) achieve its functionalityunder practical conditions (e.g. temperature and pump power), and (3)offer competitive advantage over existing technologies.

[0004] One material that has gathered much international attention iserbium (Er) doped silicon (Si). The light emission from Er-doped Sioccurs at the technological important 1.5 micron (μm) wavelength.Trivalent erbium in a proper host can have a fluorescence of 1540 nm dueto the ⁴I_(13/2)→⁴I_(15/2) intra-4f transition. This 1540 nmfluorescence occurs at the minimum absorption window of the silica-basetelecommunication fibre optics field. There is great interest in Erdoping of silicon as it holds the promise of silicon basedoptoelectronics from the marriage of the vast infrastructure and proveninformation processing capability of silicon integrated circuits withthe optoelectronics industry. Theoretical and experimental results alsosuggest that Er in Si is Auger-excited via carriers, generated eitherelectrically or optically, that are trapped at the Er-related defectsites and then recombine, and that this process can be very efficientdue to strong carrier-Er interactions. However, if this strongcarrier-Er interaction is attempted in Er-doped bulk Si, the efficiencyof the Er³⁺ luminescence is reduced at practical temperature and pumppowers.

[0005] Recently, it has been demonstrated that using silicon-richsilicon oxide (SRSO), which consists of Si nanocrystals embedded in aSiO₂ (glass) matrix, reduces many of the problems associated with bulkSi and can have efficient room temperature Er³⁺ luminescence. The Sinanocrystals act as classical sensitizer atoms that absorb incidentphotons and then transfer the energy to the Er³⁺ ion, which thenfluoresce at the 1.5 micron wavelength with the following significantdifferences. First, the absorption cross section of the Si nanocrystalsis larger than that of the Er³⁺ ions by more than 3 orders of magnitude.Second, as excitation occurs via Auger-type interaction between carriersin the Si nanocrystals and Er³⁺ ions, incident photons need not be inresonance with one of the narrow absorption bands of Er³⁺. However,existing approaches to developing such Si nanocrystals have only beensuccessful at producing concentrations of up to 0.3 atomic percent ofthe rare earth element, which is not sufficient for practicalapplications.

[0006] In general, manufacture of type IV semiconductor nanocrystalsdoped with a rare earth element is done by ion implantation of siliconions into a silicon oxide layer, followed by high temperature annealingto grow the silicon nanocrystals and to reduce the ion implantationdamage. The implantation of Si ions is followed by an ion implantationof the rare earth ions into the annealed silicon nanocrystal oxidelayer. The resulting layer is again annealed to reduce the ion implantdamage and to optically activate the rare-earth ion.

[0007] There are several problems with this method: i) it results in adecreased layer surface uniformity due to the ion implantation; ii) itrequires an expensive ion implantation step; iii) it fails to achieve auniform distribution of group IV semiconductor nanocrystals andrare-earth ions unless many implantation steps are carried out; and iv)it requires a balance between reducing the ion implant damage by thermalannealing while trying to maximise the optically active rare-earth.

[0008] To diminish the above drawbacks, Plasma Enhanced Chemical VaporDeposition (PECVD) has been utilised to make type IV semiconductornanocrystal layers. The prepared layers are then subjected to arare-earth ion implantation step and a subsequent annealing cycle toform the IV semiconductor nanocrystals, and to optically activate therare-earth ions that are doped in the nanocrystal region. Unfortunately,the layers prepared with this method are still subjected to animplantation step, which results in a decrease in surface uniformity.

[0009] Another PECVD method that has been used to obtain a doped type IVsemiconductor crystal layer consists of co-sputtering together both thegroup IV semiconductor and rare-earth metal. In this method, the groupIV semiconductor and a rare-earth metal are placed into a vacuum chamberand exposed to an Argon ion beam. The argon ion beam sputters off thegroup IV semiconductor and the rare-earth metal, both of which aredeposited onto a silicon wafer. The film formed on the silicon wafer isthen annealed to grow the nanocrystals and to optically activate therare-earth ions. As the rare earth metal is in solid form, the argon ionbeam (plasma) is only able to slowly erode the rare earth, which leadsto a low concentration of rare earth metal in the deposited film. Whilehigher plasma intensity could be used to more quickly erode the rareearth metal and increase the rare earth concentration in the film, ahigher intensity plasma damages the film or the group IV semiconductorbefore it is deposited. The plasma intensity is therefore kept low topreserve the integrity of the film, therefore limiting the rare earthconcentration in the film. The doped group IV semiconductor nanocrystallayers made through this method have the drawbacks that: i) the layerdoes not have a very uniform distribution of nanocrystals and rare-earthions, ii) the layer suffers from upconversion efficiency losses due torare-earth clustering in the film, and iii) the concentration of rareearth metal in the layer is limited by the plasma intensity, which iskept low to avoid damaging the layer.

[0010] The concentration of the rare earth element in semiconductornanocrystal layers is preferably as high as possible, as the level ofphotoelectronic qualities of the film, such as photoluminescence, isproportional to the concentration. One problem encountered when a highconcentration of rare earth element is present within the semiconductorlayer is that when two rare earth metals come into close proximity withone another, a quenching relaxation interaction occurs that reduces thelevel of photoelectronic dopant response observed. The concentration ofrare earth element within a semiconductor film is thus balanced to be ashigh as possible to offer the most fluorescence, but low enough to limitthe quenching interactions.

SUMMARY OF THE INVENTION

[0011] In one aspect, the present invention provides a dopedsemiconductor nanocrystal layer, the doped semiconductor nanocrystallayer comprising (a) a group IV oxide layer which is free of ionimplantation damage, (b) from 30 to 50 atomic percent of a semiconductornanocrystal distributed in the group IV semiconductor oxide layer, and(c) from 0.5 to 15 atomic percent of one or more rare earth element, theone or more rare earth element being (i) dispersed on the surface of thesemiconductor nanocrystal and (ii) distributed substantially equallythrough the thickness of the group IV oxide layer.

[0012] In another aspect, the present invention provides a semiconductorstructure comprising a substrate, on which substrate is deposited one ormore of the doped semiconductor nanocrystal layer described above.

[0013] In another aspect, the present invention provides a process forpreparing a doped semiconductor nanocrystal layer, the processcomprising:

[0014] (a) subjecting a target comprising a mixture of (i) a powderedgroup IV binding agent, (ii) a powdered semiconductor selected from agroup IV semiconductor, a group II-VI semiconductor and a group III-Vsemiconductor, and (iii) a powdered rare earth element, the rare earthelement being present in concentration of 0.5 to 15 atomic percent, to apulse laser deposition procedure to deposit a semiconductor rich groupIV oxide layer doped with a rare earth element, and

[0015] (b) annealing the semiconductor rich group IV oxide layer dopedwith a rare earth element at a temperature of from 600° C. to 1000° C.

[0016] In another aspect, the present invention provides a process forpreparing a doped semiconductor nanocrystal layer, the processcomprising:

[0017] (a) introducing (i) a gaseous mixture of a group IV elementprecursor and molecular oxygen, and (ii) a gaseous rare earth elementprecursor, in a plasma stream of a Plasma Enhanced chemical VaporDeposition (PECVD) instrument to form a semiconductor rich group IVoxide layer doped with a rare earth element, and

[0018] (b) annealing the semiconductor rich group IV oxide layer dopedwith a rare earth element at a temperature of from 600° C. to 1000° C.

[0019] The above and other objects, features and advantages of thepresent invention will become apparent from the following descriptionwhen taken in conjunction with the accompanying figures which illustratepreferred embodiments of the present invention by way of example.

DESCRIPTION OF THE FIGURES

[0020] Embodiments of the invention will be discussed with reference tothe following Figures:

[0021]FIG. 1 is a diagram of a semiconductor structure comprising asubstrate, a doped semiconductor nanocrystal layer, and a currentinjection layer;

[0022]FIG. 2 is a diagram of a superlattice semiconductor structurecomprising a substrate and alternating doped semiconductor nanocrystallayers and dielectric layers; and

[0023]FIG. 3 is a diagram of a Pulse Laser Deposition apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] Doped Semiconductor Nanocrystal Layer

[0025] The doped semiconductor nanocrystal layer of the inventioncomprises a group IV oxide layer in which is distributed semiconductornanocrystals. The group IV element used to prepare the layer ispreferably selected from silicon, germanium, tin and lead, and the groupIV semiconductor oxide layer is more preferably silicon dioxide. Thegroup IV oxide layer preferably has a thickness of from 1 to 2000 nm,for example of from 80 to 2000 nm, from 100 to 250 nm, from 30 to 50 nm,or from 1 to 10 nm.

[0026] The semiconductor nanocrystals that are dispersed within thegroup IV semiconductor oxide layer are preferably the nanocrystal of agroup IV semiconductor, e.g. Si or Ge, of a group II-VI semiconductor,e.g. ZnO, ZnS, ZnSe, CaS, CaTe or CaSe, or of a group III-Vsemiconductor, e.g. GaN, GaP or GaAs. The nanocrystals are preferablyfrom 1 to 10 nm in size, more preferably from 1 to 3 nm in size, andmost preferably from 1 to 2 nm in size. Preferably, the nanocrystals arepresent within the group IV semiconductor oxide layer in a concentrationof from 30 to 50 atomic percent, more preferably in a concentration of37 to 47 atomic percent, and most preferably in a concentration of from40 to 45 atomic percent.

[0027] The one or more rare earth element that is dispersed on thesurface of the semiconductor nanocrystal can be selected to be alanthanide element, such as cerium, praseodymium, neodymium, promethium,gadolinium, erbium, thulium, ytterbium, samarium, dysprosium, terbium,europium, holmium, or lutetium, or it can be selected to be an actinideelement, such as thorium. Preferably, the rare earth element is selectedfrom erbium, thulium, and europium. The rare earth element can, forexample, take the form of an oxide or of a halogenide. Of thehalogenides, rare earth fluorides are preferred as they display moreintense fluorescence due to field distortions in the rare earth-fluoridematrix caused by the high electronegativity of fluorine atoms. Mostpreferably, the rare earth element is selected from erbium oxide, erbiumfluoride, thulium oxide, thulium fluoride, europium oxide and europiumfluoride.

[0028] The one or more rare earth element is preferably present in thegroup IV semiconductor oxide layer in a concentration of 0.5 to 15atomic percent, more preferably in a concentration of 5 to 15 atomicpercent and most preferably in a concentration of 10 to 15 atomicpercent. While such a high concentration of rare earth element has ledto important levels of quenching reactions in previous dopedsemiconductor materials, the doped semiconductor nanocrystal layer ofthe present invention can accommodate this high concentration as therare earth element is dispersed on the surface of the semiconductornanocrystal, which nanocrystal offers a large surface area. The reducedamount of quenching reactions between the rare earth element and theproximity of the rare earth element to the semiconductor nanocrystalprovide the basis for a doped semiconductor nanocrystal layer thatoffers improved optoelectronic properties.

[0029] Semiconductor Structure

[0030] Using the doped semiconductor nanocrystal layer described above,a multitude of semiconductor structures can be prepared. For example, asemiconductor structure is shown in FIG. 1, in which one or more layers33 of the doped semiconductor nanocrystal layer are deposited on asubstrate 31.

[0031] The substrate on which the semiconductor nanocrystal layer isformed is selected so that it is capable of withstanding temperatures ofup to 1000° C. Examples of suitable substrates include silicon wafers orpoly silicon layers, either of which can be n-doped or p-doped (forexample with 1×10²⁰ to 5×10²¹ of dopants per cm³), fused silica, zincoxide layers, quartz and sapphire substrates. Some of the abovesubstrates can optionally have a thermally grown oxide layer, whichoxide layer can be of up to about 2000 nm in thickness, a thickness of 1to 20 nm being preferred. The thickness of the substrate is notcritical, as long as thermal and mechanical stability is retained.

[0032] The semiconductor structure can comprise a single or multipledoped semiconductor nanocrystal layers, each layer having anindependently selected composition and thickness. By using layers havingdifferent rare earth elements, a multi-color emitting structure can beprepared. For example, combining erbium, thulium and europium in asingle semiconductor structure provides a structure that can fluoresceat the colors green (erbium), blue (thulium), and red (europium).

[0033] When two or more doped semiconductor nanocrystal layers are usedin a single semiconductor structure, the layers can optionally beseparated by a dielectric layer. Examples of suitable dielectric layersinclude silicon dioxide, silicon nitrite and silicon oxy nitrite. Thesilicon dioxide dielectric layer can also optionally comprisesemiconductor nanocrystals. The dielectric layer preferably has athickness of from 1 to 10 nm, more preferably of 1 to 3 nm and mostpreferably of about 1.5 nm. The dielectric layer provides an efficienttunnelling barrier, which is important for obtaining high luminosityfrom the semiconductor structure.

[0034] The semiconductor structure can also have an Indium Tin Oxide(ITO) current injection layer (34) overtop the one or more dopedsemiconductor nanocrystal layers. The ITO layer preferably has athickness of from 150 to 300 nm. Preferably, the chemical compositionand the thickness of the ITO layer is such that the semiconductorstructure has a conductance of from 30 to 70 ohms cm.

[0035] The thickness of the semiconductor structure is preferably 2000nm or less, and the thickness will depend on the thickness of thesubstrate, the number and thickness of the doped semiconductornanocrystal layers present, the number and the thickness of the optionaldielectric layers, and the thickness of the optional ITO layer.

[0036] One type of preferred semiconductor structure provided by anembodiment of the present invention is a superlattice structure, shownby way of example in FIG. 2, which structure comprises multiple layersof hetero-material 20 on a substrate 11. Multiple doped semiconductornanocrystals layers having a thickness of from 1 nm to 10 nm aredeposited on the substrate 12 and 14, and the doped semiconductornanocrystals layers can comprise the same or different rare earthelements. Optionally, the doped semiconductor nanocrystal layers areseparated by dielectric layers 13 of about 1.5 nm in thickness, and anITO current injection layer (not shown) can be deposited on top of themultiple layers of the superlattice structure. There is no maximumthickness for the superlattice structure, although a thickness of from250 to 2000 nm is preferred and a thickness of from 250 to 750 nm ismore preferred.

[0037] Preparation of the Doped Semiconductor Nanocrystal Layer

[0038] The preparation of the doped semiconductor nanocrystal layercomprises the following two general steps:

[0039] (a) the simultaneous deposition of a semiconductor rich group IVoxide layer and of one or more rare earth element; and

[0040] (b) the annealing of the semiconductor rich group IV oxide layerprepared in (a) to form semiconductor nanocrystals.

[0041] The semiconductor rich group IV oxide layer comprises a group IVoxide layer, which group IV oxide is preferably selected from SiO₂ orGeO₂, in which group IV oxide layer is dispersed a rare earth elementand a semiconductor, which semiconductor can be the same as, ordifferent than, the semiconductor that forms the group IV oxide layer.

[0042] By “semiconductor rich”, it is meant that an excess ofsemiconductor is present, which excess will coalesce to formnanocrystals when the semiconductor rich group IV oxide layer isannealed. Since the rare earth element is dispersed within the oxidelayer when the nanocrystals are formed, the rare earth element becomesdispersed on the surface of the semiconductor nanocrystals uponnanocrystal formation.

[0043] Since the semiconductor rich group IV oxide layer and the one ormore rare earth element are deposited simultaneously, ion implantationof the rare earth element is avoided. As such, the group IV oxide layersurface is free of the damage associated with an implantation process.Also, since the rare earth element is deposited at the same time as thesemiconductor rich group IV oxide layer, the distribution of the rareearth element is substantially constant through the thickness of thegroup IV oxide layer.

[0044] The deposition of the semiconductor rich group IV oxide layerdoped with one or more rare earth elements is preferably carried out byPlasma-Enhanced Chemical Vapor Deposition (PECVD) or by Pulse LaserDeposition (PLD). The above two methods each have their respectiveadvantages for preparing the semiconductor rich group IV oxide layerdoped with one or more rare earth elements, and the methods aredescribed below.

[0045] Pulse Laser Deposition

[0046] Pulse laser deposition is advantageous for the deposition of thesemiconductor rich group IV oxide layer doped with one or more rareearth elements as it permits the deposition of a wide variety ofsemiconductors and a wide variety of rare earth elements.

[0047] Referring now to FIG. 3, which shows by way of a diagram atypical set up of a pulse laser deposition apparatus, the pulse laserdeposition apparatus consists of a large chamber 41, which can beevacuated down to at least 10⁻⁷ bars or pressurized with up to 1atmosphere of a gas such as oxygen, nitrogen, helium, argon, hydrogen orcombinations thereof. The chamber has at least one optical port 42 inwhich a pulse laser beam 45 can be injected to the chamber and focuseddown onto a suitable target 44. The target is usually placed on acarrousel 43 that allows the placement of different target samples intothe path of the pulse laser focus beam. The carrousel is controlled sothat multiple layers of material can be deposited by the pulse laserablation of the target. The flux of the focused pulse laser beam isadjusted so that the target ablates approximately 0.1 nm of thickness ofmaterial on a substrate 47, which can be held perpendicular to thetarget and at a distance of 20 to 75 millimetres above the target. Thisflux for instance is in the range of 0.1 to 20 joules per square cm for248 nm KrF excimer laser and has a pulse width of 20-45 nanosecondduration. The target can be placed on a scanning platform so that eachlaser pulse hits a new area on the target, thus giving a fresh surfacefor the ablation process. This helps prevent the generation of largeparticles, which could be ejected in the ablation plume 46 and depositedon to the substrate. The substrate is usually held on a substrate holder48, which can be heated from room temperature up to 1000° C. and rotatedfrom 0.1 to 30 RPM depending on the pulse rate of the pulse laser, whichin most cases is pulsed between 1-10 Hz. This rotation of the substrateprovides a method of generating a uniform film during the depositionprocess. The laser is pulsed until the desired film thickness is met,which can either be monitored in real time with an optical thicknessmonitor or quartz crystal microbalance or determined from a calibrationrun in which the thickness is measured from a given flux and number ofpulses. Pulse laser deposition can be used for depositing layers of from1 to 2000 nm in thickness.

[0048] For the preparation of a semiconductor rich group IV oxide layerdoped with one or more rare earth elements, the target that is ablatedis composed of mixture of a powdered group IV binding agent, a powderedsemiconductor that will form the nanocrystal, and a powdered rare earthelement. The ratio of the various components found in the dopedsemiconductor nanocrystal layer is decided at this stage by controllingthe ratio of the components that form the target. Preferably, themixture is placed in a hydraulic press and pressed into a disk of 25 mmdiameter and 5 mm thickness with a press pressure of at least 500 Psiwhile being heated to 700° C. The temperature and pressure can beapplied, for example, for one hour under reduced pressure (e.g. 10⁻³bars) for about one hour. The press pressure is then reduced and theresulting target is allowed to cool to room temperature.

[0049] The group IV binding agent can be selected to be a group IV oxide(e.g. silicon oxide, germanium oxide, tin oxide or lead oxide), oralternatively, it can be selected to be a group IV element (e.g.silicon, germanium, tin or lead). When the group IV binding agent is agroup IV oxide, the binding agent, the semiconductor and the rare earthelement are combined to form the target, and the pulse laser depositionis carried out in the presence of any one of the gases listed above. Ifa group IV element is used as the group IV binding agent instead, thepulse laser deposition is carried out under an oxygen atmosphere,preferably at a pressure of from 1×10⁻⁴ to 5×10⁻³ bar, to transform someor all of the group IV element into a group IV oxide during the laserdeposition process. When the semiconductor element which is to form thenanocrystals is selected to be a group II-VI semiconductor (e.g. ZnO,ZnS, ZnSe, CaS, CaTe or CaSe) or a group III-V semiconductor (e.g. GaN,GaP or GaAs), the oxygen concentration is kept high to insure that allof the group IV element is fully oxidized. Alternatively, if thenanocrystals to be formed comprise the same group IV semiconductorelement that is being used as the binding agent, the oxygen pressure isselected so that only part of the group IV element is oxidized. Theremaining non-oxidized group IV element can then coalesce to formnanocrystals when the prepared semiconductor rich group IV oxide layeris annealed.

[0050] The powdered rare earth element that is used to form the targetis preferably in the form of a rare earth oxide or of a rare earthhalogenide. As mentioned above, the rare earth fluoride is the mostpreferred of the rare earth halogenides.

[0051] Pulse laser deposition is useful for the subsequent deposition oftwo or more different layers. Multiple targets can be placed on thecarrousel and the pulse laser can be focussed on different targetsduring the deposition. Using this technique, layers comprising differentrare earth elements can be deposited one on top of the other to preparesemiconductor structures as described earlier. Different targets canalso be used to deposit a dielectric layer between the semiconductorrich group IV oxide layers, or to deposit a current injection layer ontop of the deposited layers. Pulse laser deposition is the preferredmethod for preparing the superlattice semiconductor structure describedabove.

[0052] Preparation of the semiconductor rich group IV oxide layer dopedwith one or more rare earth elements can of course be carried out withdifferent pulse laser deposition systems that are known in the art, theabove apparatus and process descriptions being provided by way ofexample.

[0053] Plasma Enhanced Chemical Vapor Deposition

[0054] PECVD is advantageous for the deposition of the semiconductorrich group IV oxide layer doped with one or more rare earth element, asit permits the rapid deposition of the layer. The thickness of thesemiconductor rich group IV oxide layer doped with one or more rareearth element prepared with PECVD is 10 nm or greater, more preferablyfrom 10 to 2000 nm.

[0055] Formation of a non-doped type IV semiconductor nanocrystal layerthrough chemical vapor deposition has been described, for example, by J.Sin, M. Kim, S. Seo, and C. Lee [Applied Physics Letters, (1998), Volume72, 9, 1092-1094], the disclosure of which is hereby incorporated byreference.

[0056] In this embodiment, the doped semiconductor nanocrystal layer isprepared by incorporating a rare-earth precursor into the PECVD streamabove the receiving heated substrate on which the semiconductor film isgrown. PECVD can be used to prepare the doped semiconductor nanocrystallayer where the semiconductor nanocrystal is a silicon or a germaniumnanocrystal, and where the rare earth element is a rare earth oxide.

[0057] In the PECVD process, a group IV element precursor is mixed withoxygen to obtain a gaseous mixture where there is an atomic excess ofthe group IV element. An atomic excess is achieved when the ratio ofoxygen to group IV element is such that when a group IV dioxide compoundis formed, there remains an excess amount of the group IV element. Thegaseous mixture is introduced within the plasma stream of the PEVCDinstrument, and the silicon and the oxygen are deposited on a substrateas a group IV dioxide layer in which a group IV atomic excess is found.It is this excess amount of the group IV element that coalesces duringthe annealing step to form the group IV nanocrystal. For example, toprepare a silicon dioxide layer in which silicon nanocrystals isdispersed, a silicon rich silicon oxide (SRSO) layer is deposited on thesubstrate.

[0058] The group IV element precursor can contain, for example, silicon,germanium, tin or lead, of which silicon and germanium are preferred.The precursor itself is preferably a hydride of the above elements. Aparticularly preferred group IV element precursor is silane (SiH₄).

[0059] The ratio (Q) of group IV element precursor to oxygen can beselected to be from 3:1 to 1:2. If an excess of group IV elementprecursor hydride is used, the deposited layer can contain hydrogen, forexample up to approximately 10 atomic percent hydrogen. The ratio of theflow rates of the group IV element precursor and of oxygen can be kept,for example, between 2:1 and 1:2.

[0060] Also introduced to the plasma stream is a rare earth elementprecursor, which precursor is also in the gaseous phase. The rare earthprecursor is added to the plasma stream at the same time as the group IVelement precursor, such that the rare earth element and the group IVelement are deposited onto the substrate simultaneously. Introduction ofthe rare earth precursor as a gaseous mixture provides better dispersionof the rare earth element within the group IV layer. Preferably,presence of oxygen in the plasma stream and in the deposited layer leadsto the deposition of the rare earth element in the form of a rare earthoxide.

[0061] The rare earth element precursor comprises one or more ligands.The ligand can be neutral, monovalent, divalent or trivalent.Preferably, the ligand is selected so that when it is coordinated withthe rare earth element, it provides a compound that is volatile, i.e.that enters the gaseous phase at a fairly low temperature, and withoutchanging the chemical nature of the compound. The ligand also preferablycomprises organic components that, upon exposure to the plasma in thePECVD apparatus, will form gaseous by-products that can be removedthrough gas flow or by reducing the pressure within the PECVD apparatus.When the organic components of the ligand are conducive to producingvolatile by-products (e.g. CO₂, O₂) less organic molecules areincorporated into the deposited layer. Introduction of organic moleculesinto the deposited layer is generally not beneficial, and the presenceof organic molecules is sometimes referred to as semiconductorpoisoning.

[0062] Suitable ligands for the rare earth element can include acetatefunctions, for example 2,2,6,6-tetramethyl-3,5-heptanedione,acetylacetonate, flurolacetonate,6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione,i-propylcyclopentadienyl, cyclopentadienyl, and n-butylcyclopentadienyl.Preferred rare earth metal precursor includetris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III), erbium (III)acetylacetonate hydrate, erbium (III) flurolacetonate,tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)erbium(III), tris(i-propylcyclopentadienyl)erbium (III),Tris(cyclopentadienyl)erbium (III), andtris(n-butylcyclopentadienyl)erbium (III). A particularly preferred rareearth element precursor is tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium(III) (Er⁺³ [(CH₃)₃CCOCH═COC(CH₃)₃]₃), which is also referred toas Er⁺³ (THMD)₃.

[0063] If the rare earth element precursor is not in the gaseous phaseat room temperature, it must be transferred to the gaseous phase, forexample, by heating in an oven kept between 80° C. and 110° C. Thegaseous rare earth element precursor is then transferred to the plasmastream with an inert carrier gas, such as argon. The gaseous rare earthelement precursor is preferably introduced to the plasma at a positionthat is below a position where the group IV element containing compoundis introduced to the plasma. Use can be made of a dispersion mechanism,for example a dispersion ring, to assist in the dispersion of thegaseous rare earth element precursor in the plasma.

[0064] In order to obtain a more even deposition of the doped type IVoxide layer, the substrate can be placed on a sceptre that rotatesduring deposition. A circular rotation of about 3 rpm is suitable forincreasing the uniformity of the layer being deposited.

[0065] An Electron Cyclotron Resonated (ECR) reactor is suitable forproducing the plasma used in the PECVD method described above. ECR is aparticular method of generating plasma, where the electrons have aspiral motion caused by a magnetic field, which allows a high density ofions in a low-pressure region. The high ion density with low pressure isbeneficial for deposition, as the rare earth metal precursor can bestripped of its organic components and incorporated uniformly and in ahigh concentration. The plasma used in the PECVD method can comprise,for example, argon, helium, neon or xenon, of which argon is preferred.

[0066] The PECVD method is carried out under a reduced pressure, forexample 1×10⁻⁷ torr, and the deposition temperature, microwave power andscepter bias can be kept constant. Suitable temperature, microwave andscepter bias values can be selected to be, for example, 300° C., 400 Wand −200 V_(DC), respectively.

[0067] The semiconductor rich group IV oxide layer doped with one ormore rare earth element can be grown at different rates, depending onthe parameters used. A suitable growth rate can be selected to be about60 nm per minute, and the semiconductor rich group IV oxide layer canhave a thickness of from 10 to 2000 nm, more preferably of from 100 to250 nm.

[0068] Preparation of the semiconductor rich group IV oxide layer dopedwith one or more rare earth elements can of course be carried out withdifferent plasma enhanced chemical vapor deposition systems that areknown in the art, the above apparatus and process descriptions beingprovided by way of example.

[0069] Annealing Step

[0070] After the semiconductor rich group IV oxide layer doped with oneor more rare earth element has been prepared, the doped type IV oxidelayer is annealed, optionally under flowing nitrogen (N₂), in a RapidThermal Anneal (RTA) furnace, at from about 600° C. to about 1000° C.,more preferably from 800° C. to 950° C., from 5 minutes to 30 minutes,more preferably from 5 to 6 minutes. It is during the annealing stepthat the atomic excess of semiconductor is converted into semiconductornanocrystals.

[0071] When PECVD is used to prepare the semiconductor rich group IVoxide layer doped with one or more rare earth element, the annealingstep can also be carried out under an oxygen atmosphere to insureoxidation of the rare earth element, or under a reduced pressure inorder to facilitate the removal of any volatile by-products that mightbe produced.

[0072] The amount of excess semiconductor in the group IV oxide layerand the anneal temperature dictate the size and the density of thesemiconductor nanocrystal present in the final doped semiconductornanocrystal layer.

[0073] Since the rare earth element is well dispersed through thedeposited group IV semiconductor oxide layer, when the nanocrystals areformed during the annealing step, the rare earth element becomeslocalised on the surface of the nanocrystals. Since the nanocrystalsprovide a large surface area on which the rare earth element can bedispersed, the concentration of the rare earth element can be quiteelevated, while retaining good photoelectronic properties.

[0074] The following examples are offered by way of illustration and notby way of limitation.

EXAMPLES Example 1

[0075] Silane (SiH₄) and oxygen (O₂) are added to an argon plasma streamproduced by an Electron Cyclotron Resonated (ECR) reactor via dispersionring. The ratio (Q) of silane to oxygen has been varied between3:1,1.7:1,1.2:1,1:1.9, and 1:2. An erbium precursor(Tris(2,2,6,6-tetramethyl-3,5-heptanedionato) erbium(III) [Er⁺³(THMD)₃]) is placed in a stainless steel oven held between 90 and 110°C.

[0076] A carrier gas of Ar is used to transport the Er precursor fromthe oven through a precision controlled mass-flow controller to adispersion ring below the Silane injector and above the heatedsubstrate. The instrument pressure is kept at about 1×10⁻⁷ torr. Thesubstrates used are either fuse silica or silicon wafers on which isthermally grown an oxide layer of 2000 nm thickness. The depositiontemperature, the microwave power and the sceptre bias are kept constantat 300° C., 400 W and −200 V_(DC). The SiH₄, Ar flow rates were adjustedwhile keeping the O₂ flow rate at 20 militorr sec⁻¹ for the variousexcess silicon content. The Er/Ar flow rate was adjusted to the vaporpressure generated by the temperature controlled oven for the desirederbium concentration. The film is grown at a rate of 60 nm per minuteand thickness has been grown from 250 nm to 2000 nm thick. The scepterwas rotated at 3 rpm during the growth to help in uniformity of film.After deposition, the samples are annealed at 950° C.-1000° C. for 5-6minutes under flowing nitrogen (N₂) in a Rapid Thermal Anneal (RTA)furnace.

Example 2

[0077] An ablation target is fabricated by combining powdered silicon,powdered silicon dioxide and powdered erbium oxide, the prepared powdermixture comprising 45% silicon, 35% silicon oxide and 20% erbium oxide.Each powder component has a size of about 300 mesh. The mixture isplaced into a ball mill and ground for approximately 5 to 10 minutes.The mixture is then placed into a 25 mm diameter by 7 mm thick mould,placed into a hydraulic press, and compressed for 15 minutes at 500 psi.The obtained target is then placed into an annealing furnace and heatedto 1200° C. in a forming gas atmosphere of 5% H₂ and 95% N₂ for 30minutes. The Target is cooled down to room temperature and then regroundin a ball mill for ten minutes. The mixture is then again placed in amould, compressed and annealed as described above. The obtained targetis placed onto a target holder inside a vacuum chamber. A siliconsubstrate [n-type, <110> single crystal, 0.1-0.05 Ωcm conductivity] of50 mm diameter and 0.4 cm thickness is placed on a substrate holderparallel to and at a distance of 5.0 cm above the surface of the target.The substrate is placed onto a substrate support that is heated at 500°C., and the substrate is rotated at a rate of 3 rpm during thedeposition. The vacuum chamber is evacuated to a base pressure of 1×10⁻⁷torr and then back filled with 20×10⁻³ torr of Ar. An excimer laser (KrF248 nm) is focused on to the target at an energy density of about 10Jcm⁻² and at a glancing angle of 40° to the vertical axis, such that a0.1 nm film is generated per pulse. The target is rotated at 5 rpmduring deposition in order to have a fresh target surface for eachablation pulse. After a 100 nm layer is deposited on the substrate, thenewly deposited film is annealed at temperature of from 900° C. to 950°C. for 5 minutes to form silicon nanocrystals in the Silicon RichSilicon Oxide (SRSO).

[0078] The substrate is reintroduced in the vacuum chamber, and thetarget is replaced with an Indium Tin Oxide (ITO) target. The atmosphereinside the vacuum chamber is set to 2×10³ torr of O₂, and the substrateis heated to 500° C. and rotated at 3 rpm. A 100 nm ITO layer isdeposited on top of the annealed rare earth doped SRSO film.

[0079] All publications, patents and patent applications cited in thisspecification are herein incorporated by reference as if each individualpublication, patent or patent application were specifically andindividually indicated to be incorporated by reference. The citation ofany publication is for its disclosure prior to the filing date andshould not be construed as an admission that the present invention isnot entitled to antedate such publication by virtue of prior invention.

[0080] Although the foregoing invention has been described in somedetail by way of illustration and example for purposes of clarity ofunderstanding, it is readily apparent to those of ordinary skill in theart in light of the teachings of this invention that certain changes andmodifications may be made thereto without departing from the spirit orscope of the appended claims.

[0081] It must be noted that as used in this specification and theappended claims, the singular forms “a”, “an”, and “the” include pluralreference unless the context clearly dictates otherwise. Unless definedotherwise all technical and scientific terms used herein have the samemeaning as commonly understood to one of ordinary skill in the art towhich this invention belongs.

We claim:
 1. A doped semiconductor nanocrystal layer comprising (a) agroup IV oxide layer which is free of ion implantation damage, (b) from30 to 50 atomic percent of a semiconductor nanocrystal distributed inthe group IV oxide layer, and (c) from 0.5 to 15 atomic percent of oneor more rare earth element, the one or more rare earth element being (i)dispersed on the surface of the semiconductor nanocrystal and (ii)distributed substantially equally through the thickness of the group IVoxide layer.
 2. A doped semiconductor nanocrystal layer according toclaim 1, wherein the group IV oxide layer comprises silicon dioxide orgermanium dioxide.
 3. A doped semiconductor nanocrystal layer accordingto claim 1, wherein the group IV oxide layer has a thickness of from 1to 2000 nm.
 4. A doped semiconductor nanocrystal layer according toclaim 1, wherein the group IV oxide layer has a thickness of from 80 to2000 nm.
 5. A doped semiconductor nanocrystal layer according to claim1, wherein the group IV oxide layer has a thickness of from 100 to 250nm.
 6. A doped semiconductor nanocrystal layer according to claim 1,wherein the group IV oxide layer has a thickness of from 1 to 10 nm. 7.A doped semiconductor nanocrystal layer according to claim 1, whereinthe semiconductor nanocrystal is a group IV semiconductor, a group II-VIsemiconductor or a group III-V semiconductor.
 8. A doped semiconductornanocrystal layer according to claim 7, wherein the group IVsemiconductor is selected from Si, Ge, Sn and Pb.
 9. A dopedsemiconductor nanocrystal layer according to claim 7, wherein the groupII-VI semiconductor is selected from ZnO, ZnS, ZnSe, CaS, CaTe and CaSe.10. A doped semiconductor nanocrystal layer according to claim 7,wherein the group III-I semiconductor is selected from GaN, GaP andGaAs.
 11. A doped semiconductor nanocrystal layer according to claim 1,wherein the concentration of semiconductor nanocrystals in the group IVoxide layer is from 37 to 47 atomic percent.
 12. A doped semiconductornanocrystal layer according to claim 1, wherein the concentration ofsemiconductor nanocrystals in the group IV oxide layer is from 40 to 45atomic percent.
 13. A doped semiconductor nanocrystal layer according toclaim 1, wherein the semiconductor nanocrystals are from 1 to 10 nm insize.
 14. A doped semiconductor nanocrystal layer according to claim 1,wherein the semiconductor nanocrystals are from 1 to 3 nm in size.
 15. Adoped semiconductor nanocrystal layer according to claim 1, wherein thesemiconductor nanocrystals are from 1 to 2 nm in size.
 16. A dopedsemiconductor nanocrystal layer according to claim 1, wherein the rareearth element is selected from cerium, praseodymium, neodymium,promethium, gadolinium, erbium, thulium, ytterbium, samarium,dysprosium, terbium, europium, holmium, lutetium, and thorium.
 17. Adoped semiconductor nanocrystal layer according to claim 16, wherein therare earth element is selected from erbium, thulium and europium.
 18. Adoped semiconductor nanocrystal layer according to claim 1, wherein therare earth element is in the form of an oxide or a halogenide.
 19. Adoped semiconductor nanocrystal layer according to claim 18, wherein thehalogenide is a fluoride.
 20. A doped semiconductor nanocrystal layeraccording to claim 1, wherein the rare earth concentration is from 5 to15 atomic percent.
 21. A doped semiconductor nanocrystal layer accordingto claim 1, wherein the rare earth concentration is from 10 to 15 atomicpercent.
 22. A semiconductor structure comprising a substrate, on whichsubstrate is deposited one or more doped semiconductor nanocrystallayers according to claim
 1. 23. A semiconductor structure according toclaim 22, wherein the substrate is selected from a silicon wafers or apoly silicon layer, either of which can be optionally n-doped orp-doped, and a layer of fused silica, zinc oxide, quartz or sapphire.24. A semiconductor structure according to claim 22, wherein thesemiconductor structure comprises one or more dielectric layer.
 25. Asemiconductor structure according to claim 24, wherein the dielectriclayer comprise silicon oxide, silicon nitrite or silicon oxy nitrite.26. A semiconductor structure according to claim 24, wherein thedielectric layer has a thickness of 1 to 10 nm.
 27. A semiconductorstructure according to claim 24, wherein the dielectric layer has athickness of 1 to 3 nm.
 28. A semiconductor structure according to claim24, wherein the dielectric layer has a thickness of about 1.5 nm.
 29. Asemiconductor structure according to claim 22, wherein the semiconductorstructure comprises a current injection layer.
 30. A semiconductorstructure according to claim 29, wherein the current injection layer isan indium tin oxide layer.
 31. A semiconductor structure according toclaim 22, wherein the semiconductor structure has a thickness of 2000 nmor less.
 32. A process for preparing a doped semiconductor nanocrystallayer, the process comprising: (a) subjecting a target comprising amixture of (i) a powdered group IV binding agent, (ii) a powderedsemiconductor selected from a group IV semiconductor, a group II-VIsemiconductor and a group III-V semiconductor, and (iii) a powdered rareearth element, the rare earth element being present in concentration of0.5 to 15 atomic percent, to a pulse laser deposition procedure todeposit a semiconductor rich group IV oxide layer doped with a rareearth element, and (b) annealing the semiconductor rich group IV oxidelayer doped with a rare earth element at a temperature of from 600° C.to 1000° C.
 33. A process according to claim 32, wherein the powderedgroup IV binding agent is selected from silicon oxide, germanium oxide,lead oxide and tin oxide.
 34. A process according to claim 32, whereinthe powdered group IV binding agent is selected from silicon, germanium,lead and tin, and wherein the pulse laser deposition procedure iscarried out under an oxygen atmosphere.
 35. A process according to claim34, wherein the oxygen atmosphere has a pressure suitable to obtain thesemiconductor rich group IV oxide layer with 30 to 50 atomic percent ofexcess semiconductor.
 36. A process according to claim 32, wherein thegroup IV semiconductor is selected from Si, Ge, Sn and Pb.
 37. A processaccording to claim 32, wherein the group II-VI semiconductor is selectedfrom ZnO, ZnS, ZnSe, CaS, CaTe and CaSe.
 38. A process according toclaim 32, wherein the group III-I semiconductor is selected from GaN,GaP and GaAs.
 39. A process according to claim 32, wherein the powderedrare earth element is selected from cerium, praseodymium, neodymium,promethium, gadolinium, erbium, thulium, ytterbium, samarium,dysprosium, terbium, europium, holmium, lutetium, and thorium.
 40. Aprocess according to claim 32, wherein the powdered rare earth elementis selected from erbium, thulium and europium.
 41. A process accordingto claim 32, wherein the powdered rare earth element is in the form ofan oxide or a halogenide.
 42. A process according to claim 40, whereinthe halogenide is a fluoride.
 43. A process according to claim 32,wherein powdered rare earth element concentration is from 5 to 15 atomicpercent.
 44. A process according to claim 32, wherein powdered rareearth element concentration is from 10 to 15 atomic percent.
 45. Aprocess according to claim 32, wherein the semiconductor rich group IVoxide layer is annealed at a temperature of from 800 to 950° C.
 46. Aprocess for preparing a doped semiconductor nanocrystal layer, theprocess comprising: (a) introducing (i) a gaseous mixture of a group IVelement precursor and molecular oxygen, and (ii) a gaseous rare earthelement precursor, in a plasma stream of a Plasma Enhanced chemicalVapor Deposition (PECVD) instrument to form a semiconductor rich groupIV oxide layer doped with a rare earth element, and (b) annealing thesemiconductor rich group IV oxide layer doped with a rare earth elementat a temperature of from 600° C. to 1000° C.
 47. A process according toclaim 46, wherein the group IV element precursor is a hydride of a groupIV element.
 48. A process according to claim 46, wherein the group IVelement precursor comprises silicon, germanium, tin or lead.
 49. Aprocess according to claim 46, wherein the group IV element precursor issilane.
 50. A process according to claim 46, wherein the ratio of thegroup IV element precursor and of the molecular oxygen is selected toobtain the semiconductor rich group IV oxide layer with 30 to 50 atomicpercent of excess semiconductor.
 51. A process according to claim 46,wherein the rare earth element precursor comprises a rare earth elementselected from cerium, praseodymium, neodymium, promethium, gadolinium,erbium, thulium, ytterbium, samarium, dysprosium, terbium, europium,holmium, lutetium, and thorium.
 52. A process according to claim 46,wherein the rare earth element precursor comprises erbium, thulium oreuropium.
 53. A process according to claim 46, wherein the rare earthelement precursor comprises a ligand selected from2,2,6,6-tetramethyl-3,5-heptanedione, acetylacetonate, flurolacetonate,6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedione,i-propylcyclopentadienyl, cyclopentadienyl, and n-butylcyclopentadienyl.54. A process according to claim 46, wherein the rare earth elementprecursor is selected from tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium(III), erbium (III) acetylacetonate hydrate, erbium (III)flurolacetonate,tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate)erbium(III), tris(i-propylcyclopentadienyl)erbium (III),Tris(cyclopentadienyl)erbium (III), andtris(n-butylcyclopentadienyl)erbium (III).
 55. A process according toclaim 46, wherein the semiconductor rich group IV oxide layer isannealed at a temperature of from 800 to 950° C.
 56. A dopedsemiconductor nanocrystal layer comprising (a) a group IV oxide layerwhich is free of ion implantation damage, (b) a semiconductornanocrystal distributed in the group IV oxide layer, and (c) one or morerare earth element, the one or more rare earth element being dispersedon the surface of the semiconductor nanocrystal.